The present invention relates to a telescopic crane of a type having a substructure, a revolving superstructure mounted to the substructure, a counterweight and a boom structure comprised of a main boom having a boom base and at least one telescope section which is received in the boom base and displaceable between retracted and extended positions.
In extended position, telescopic cranes are exposed to varying degrees of stress depending on the angular disposition. Oftentimes, the lateral deformation of the main boom in steep-incline disposition is the criterion that limits the load-carrying capability. When the angular disposition is flat or average, the loads applied in the mounting of the extended telescope sections represent a crucial criterion for the maximum load-carrying capability. In the latter case, the so-called super lift operation has been developed for torque relief.
The use of telescopic cranes with super lift operation has been known for a long time. An example is illustrated in a brochure issued by Mannesmann Demag Fördertechnik, Demag AX 1600, April 1996, pages 5, 17 and 27. A guy truss is arranged on the boom base of the main boom for placement on the boom base to increase the load-carrying capability and to reduce sag of the main boom. The guy truss is connected, on the one hand, to the foot region of the main boom via a guy rope of substantially constant length, and, on the other hand, to the head or collar of one of the inner telescope sections via a further guy rope of normally variable length. This reinforcing configuration is applicable for the base unit alone, or in conjunction with the arrangement of a fly jib in the form of a latticed tower which can be of the fixed type or luffing type. Telescopic cranes of this kind suffer shortcomings because the boom structure deforms laterally, especially in steep-incline disposition.